The Glowing Avalanches: Pumice and Scoria Flows

Time and again an eruptive cycle has been seen to grow from mild beginnings to a climax of great violence. As the conduits are widened and more pressure is removed above the feeding chamber, magma escapes with increasing rapidity, the more so since, coming from greater depths, it tends to have an ever increasing content of dissolved gases and so froths more and more forcibly into pumice and scoria.

The historic outbreak of Krakatau in 1883 commenced during May, and for approximately three months the intensity of the eruptions was weak or moderate. The well bedded character of the early products shows that the pumice fell from the air in showers. Late in August, activity culminated with stupendous eruptions of pumice in the form of glowing avalanches, accompanying and following which the summits of the old cones collapsed to form the present caldera. Similarly, the eruptions which led to the formation of the caldera of Santorin grew in intensity to the climax. So it was also at Komagatake in 1929.⁷

The same sequence of events took place at Mount Mazama. The earlier pumice fall was immediately followed by more powerful explosions in which the pumice escaped from the crater in too large volume and too rapidly to be thrown far into the air, but instead fell en masse on the upper slopes of the volcano, and swept downward into the canyons and thence far across the surrounding flats. Gaining momentum as they plunged down from the steep summit region, and being unusually mobile by reason of the continuous discharge of gas from the constituent fragments, they moved at prodigious speed. Each incandescent clot of magma in the avalanche gave off gas under high pressure; each time bombs burst in crashing together or against obstacles in their path, there were miniature explosions. Cushioned by highly compressed vapors, the heavy load was so mobile, and its momentum in falling was so great, that the flows rushed down the canyon of the Rogue River for 35 miles. Northward, they spread across Diamond Lake, 13 miles from the vent, and emptied into the headwaters of the North Umpqua River; southward, they poured down the canyons of Annie and Sun creeks; southeastward, they raced down Sand Creek, and even after reaching the foot of Mount Mazama continued across the flats for more than 10 miles; eastward, the flows poured out of the mouths of the canyons and spread no less than 25 miles from their source. Gravity and the auto-explosiveness of the heavy mass enabled it to travel these great distances, and it can hardly be doubted that the velocity of the flows in their upper parts was well over 100 miles an hour. Such was their power that they carried lumps of pumice 6 feet in diameter a distance of 20 miles.

In their mode of propulsion they were like the nuées ardentes of Mount Pelée and the "sand flow" of the Valley of Ten Thousand Smokes, Alaska. So rapidly did they rush down the steep upper slopes of Mount Mazama that they did not begin to dump their loads until they entered the canyons below or passed on to gentler gradients. Indeed, near their source they probably eroded the surface over which they moved, as the glowing avalanches of Mount Pelée, Merapi, and Komagatake are known to have done.

Before generalizing further, it seems best to discuss the main characters of the flows.

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